This invention relates generally to film-monitoring systems and, particularly, to a reloadable film canister with a replaceable spool insertable into the canister for storing and dispensing film. The spool and/or canister may be packaged as a unit, or as part of a system for indicating the diameter of film roll within the canister, for calculating and displaying the amount of film remaining in the canister, and for indicating the absence of film in the canister.
Accurate knowledge of the amount of film remaining in a film canister is important for camera systems used for computer output microfilm. These camera systems are typically connected either to a host computer, a magnetic tape drive, or some other equipment which has stored blocks of data which are to be printed on the film by the camera system. These data blocks vary in size and anywhere from a few feet to several hundred feet of film may be required to print the data. It is important to know the length of film remaining in the canister before the printing of a block of data is started so that there is enough film in the canister to print the block. This will allow the user to load a new full roll of film rather than have to splice the film in the middle of a data block.
Another reason it is important to accurately know the length of film left because some applications require that a substantial length of film be left unprinted at the very end of the film to facilitate threading into developer equipment. The accurate knowledge of length of film left allows the camera system to automatically stop when a predetermined amount of film is left and therefore prevent the loss of data due to exposure to light during the threading process.
Determining the amount of film in a film canister has either been inaccurate or inconvenient with prior art devices. One device that visually indicates on the side of the canister the amount of film left in the canister incorporates a lever mechanism which contacts the outside of the film roll. This provides only a relative reading with poor accuracy. The operator must stop the camera system and open the film bay area to read the amount of film left. This causes waste by exposing unprocessed film.
A second device is a meter-only system which allows for the display of film left information on an external device such as a CRT screen. It uses metered feed rollers to determine the amount of film removed from a canister having a predetermined starting length of film. This system simply subtracts the amount of film metered out from the known starting length. This method, due to accumulating metering errors, provides relatively poor accuracy as the canister approaches empty. The accuracy of this method also can be seriously degraded by "soft" errors of the system (hardware or software) which lose blocks of metering data. Additionally, this method is inconvenient because canisters are sometimes removed before the film in them is used up. This requires that the amount of film left in a partially used canister, as determined by the metering system, be written on the canister. The recorded length of film remaining in the canister must be manually entered into the system when that canister is inserted or reinserted.
A third device is described in U.S. Pat. No. 3,730,453, entitled "EARLY END TAPE DETECTION," issued May 1, 1973, to inventors S. E. Hotchkiss, B. H. Smith, and P. L. Stefko. This device provides a means (an output signal) for identifying when a predetermined position is reached on a tape. Each predetermined position signifies that a predetermined quantity of tape remains for use. The device detects changes in angular velocities of a tape supply reel as tape is dispensed from the reel, and produces the output signal when the changing angular velocities (expressed in terms of pulse periods) become equal to a predetermined angular velocity (reference pulse period) when the predetermined position is reached on the tape. This device does not provide for determination of the length-of-tape (or film)--left without the use of factors such as predetermined pulse periods, derived from predetermined positions, it does not have the capability to provide for continuous readout of the length of film left.
Because it works on the principle of changing angular velocity, this method requires high accuracy in spindle drive velocity, in the reference frequency, and in operation of the comparator circuitry, and creates problems in applications (such as camera systems) where the medium (tape or film) needs to start and stop frequently, accelerating and decelerating through an entire range of angular velocities. Furthermore, this device should not be used for dispensing photographic film because it provides no means for shielding the film from ambient light. Even if the device were surrounded by a light-tight enclosure, the film, most likely, could not be loaded without risk of exposure unless the lights in the room were turned off. Also, the light source for the photodetector could fog the film.